.mov files freeze during playback

I've been using FCS Pro to edit a movie, and everything went just fine, until today playback started giving me problems: video freezes after 1 second of playback, while the audio continues (and the cursor moves on on the timeline). I thought this could be a problem of FC, but even opening a .mov file from the Finder through Quicktime Player gives me the same problem. Thought maybe the computer was slowed down by something, so I used a couple of cleaning utilities, but nothing worked. I reintalled Quicktime, nothing happened.
It is very important for me to finish the editing this week, anybody has any advice on how to restore Quicktime's functionality?